One problem in feeding a top or bottom sheet of media from a stack of sheets of media is that at least the next adjacent sheet may be fed at the same time. Accordingly, various separating means have previously been suggested for separating a top sheet of a stack of sheets of media from the next adjacent sheet when the feed is from the top of the stack of sheets of media and for separating a bottom sheet of a stack of sheets of media from the next adjacent sheet when the feed is from the bottom of the stack of sheets of media.
It is known to separate a top sheet of a stack of sheets from the next adjacent sheet through using a dam, which is an element having an inclined surface in the path of the top sheet as it is fed from the stack of sheets so that its leading edge will strike the inclined surface of the element. In a printer, for example, the advancement of more than one sheet from the stack of sheets can cause jamming. Therefore, it is necessary to avoid simultaneous advancement of more than one sheet from a stack of sheets of media to a processing station such as a printer, for example.
U.S. Pat. No. 4,934,684 to Gysling discloses a complex and expensive mechanism for advancing only a top sheet from a stack of sheets. A thickness sensor assembly senses the thickness of the sheet or sheets after advancement of the sheet or sheets from the stack past an angled dam, which has a smooth inclined or angled surface and is supposed to separate the top sheet from the next adjacent sheet. If more than one sheet is sensed by the thickness sensor assembly, a signal is produced to cause a brake shoe to frictionally engage the lower surface of a group of two or more sheets to separate the sheets. This allows only the top sheet to be advanced.
When a dam has its inclined surface formed with longitudinally extending ribs, there is corrugation of a sheet of media between the substantially parallel ribs when the sheet is advanced longitudinally along the exterior surface of each of the ribs. This corrugation is due to a buckling force created by resistance to movement of the sheet by the exterior surface of each of the ribs. While this dam is usually successful in separating an uppermost or top sheet from the next adjacent sheet in a stack of sheets, it is not always successful. Thus, multiple feeding of sheets can occur as the sheets advance up the inclined, ribbed surface of the dam.
Additionally, a surface having a coefficient of friction capable of reliably separating heavy media such as envelopes and labels, for example, tends to have more multiple sheet feeding of a light or high friction media such as bond or xerographic paper, for example. This presents the problem of whether to have an inclined surface of a dam capable of reliably separating heavy media or light media. This is not desirable with a printer since a printer needs to be capable of printing both heavy and light media to have a sufficient market.